1. Field of the Invention
This invention relates to a fuel nozzle assembly for a gas turbine engine, and more particularly to such an assembly having separate air and fuel delivery passages.
2. Description of the Prior Art
The prior art will be more fully described in detail in the following Description of the Preferred Invention; however, generally speaking, a fuel nozzle assembly capable of separately delivering both air and fuel to a combustion chamber comprises a fuel delivery tube supported from one end having a fuel nozzle tip secured to the other, and an air delivery tube, also supported at one end, and generally enclosing the fuel delivery tube in a spaced relationship to define therebetween an annular air flow channel. A cap is threaded onto the free end of the air delivery tube and tightened so that a frusto-conical opening in the face thereof sealingly engages the frusto-conical surface of the nozzle tip. However, the temperature of the fuel flowing through the fuel tube is generally on the order of about 100.degree. F. whereas the temperature of the air in the space between the tubes is on the order of about 600.degree. F. This causes a differential in the axial expansion of the two tubes resulting in a gap at the normally sealing interface of the air cap and the nozzle tip. This gap provides an area where contaminates from the air flowing therethrough or carbon deposits caused by occasional reverse flow from the combustor can accumulate to prevent the gap from resealing.
As the air delivered through the assembly is primarily used only at ignition of the gas turbine engine to atomize the fuel, it is important to provide an atomizing air pattern which is predictable and delivers an atomized fuel-air mixture generally adjacent to either a flame crossover tube or the spark ignitor, or both. However, any gap between the air tube and the fuel nozzle tip provides an air leakage path that deleteriously affects the atomizing air distribution such that an unpredictable fuel-air pattern can exist, which in turn can produce erratic and unpredictable light-off characteristics.
Further, once the fuel nozzle assembly of the known prior art is assembled and mounted in the combustor of a gas turbine engine, it becomes generally impossible to mechanically clean the air delivery channel and eliminate the deposits that cause the leakage, as will be evident from the detailed description of the prior art nozzle.
Reference is made to U.S. Pat. No. 3,763,650, having a common assignee with the present application, for showing the relative position of a fuel nozzle assembly in the combustor of the gas turbine engine, and the spatial relationship of the spark ignitor thereto.